Hydrogeologic Setting and Ground Water Flow Beneath a Section of Indian River Bay, Delaware

Abstract

The small bays along the Atlantic coast of the Delmarva Peninsula (Delaware, Maryland, and Virginia) are a valuable natural resource, and an asset for commerce and recreation. These coastal bays also are vulnerable to eutrophication from the input of excess nutrients derived from agriculture and other human activities in the watersheds. Ground water discharge may be an appreciable source of fresh water and a transport pathway for nutrients entering the bays. This paper presents results from an investigation of the physical properties of the surficial aquifer and the processes associated with ground water flow beneath Indian River Bay, Delaware. A key aspect of the project was the deployment of a new technology, streaming horizontal resistivity, to map the subsurface distribution of fresh and saline ground water beneath the bay. The resistivity profiles showed complex patterns of ground water flow, modes of mixing, and submarine ground water discharge. Cores, gamma and electromagnetic‐induction logs, and in situ ground water samples collected during a coring operation in Indian River Bay verified the interpretation of the resistivity profiles. The shore‐parallel resistivity lines show subsurface zones of fresh ground water alternating with zones dominated by the flow of salt water from the estuary down into the aquifer. Advective flow produces plumes of fresh ground water 400 to 600 m wide and 20 m thick that may extend more than 1 km beneath the estuary. Zones of dispersive mixing between fresh and saline ground water develop on the upper, lower, and lateral boundaries of the plume. The plumes generally underlie small incised valleys that can be traced landward to streams draining the upland. The incised valleys are filled with 1 to 2 m of silt and peat that act as a semiconfining layer to restrict the downward flow of salt water from the estuary. Active circulation of both the fresh and saline ground water masses beneath the bay is inferred from the geophysical results and supported by geochemical data.

Flow and geochemistry of groundwater beneath a back-barrier lagoon: The subterranean estuary at Chincoteague Bay, Maryland, USA

“…The maximum drilling depth reached offshore was 23 m beneath the sediment surface. The approach to logging, sampling, and analysis used in this investigation was similar to that employed in a previous investigation in Delaware (Böhlke and Krantz, 2003;Bratton et al, 2004;Krantz et al, 2004). A brief description of field and laboratory methods is included here; more details are provided in Section A of the Supplemental Material (sections A.1 through A.4).…”

Discharge of nitrate-containing groundwater into a coastal marine environment

“…Vanek and Lee (1991) reached a comparable conclusion for the Laholm Bay in Sweden where onshore investigations were insufficient to capture the complex offshore groundwater discharge. Manheim et al (2004) and Krantz et al (2004) also used submarine resistivity surveying to map occurrences of submarine fresh groundwater in several estuaries on the east coast of the US and found a complex distribution of freshwater. Although this distribution probably could not be predicted entirely by the terrestrial geology, there was some indication that it is controlled by submerged paleodrainage systems , much like the buried channels of this study.…”

Oyster and Macroalgae Bioindicators Detect Elevated δ15N in Maryland’s Coastal Bays

“…Freshwater base flow, transporting nitrate from terrestrial recharge areas, enters Delmarva Peninsula's coastal lagoons via both groundwater (Andres 1992;Bratton et al 2004;Krantz et al 2004;Manheim et al 2004) and overland sources that include riverine (Lung 1994;Schwartz 2003) and agricultural ditches (Schmidt et al 2007). Seasonal precipitation is variable across these coastal bays (Fig.…”